Packages for housing electrical or electronic devices, such as semiconductor devices are divided into two different types including ceramic packages and plastic packages. Ceramic packages are air cavity type packages generally formed by filling the inside of a package with air so as to prevent the active surfaces of electrical or electronic device chips installed therein from being in contact with the material of the package, but plastic packages are commonly not of air cavity type. The air cavity type ceramic packages are necessary for physically weak devices, the desired functions and characteristics of which may be severely deteriorated due to contact with the material of a package, high frequency devices which are electrically sensitive, surface acoustic wave filter devices, and optical devices and C-MOS image sensor or charge coupled devices, which require a transparent window structure. However, the ceramic packages are relatively expensive and are not fit for mass production in terms of packaging operation efficiency.
Accordingly, there have been various attempts to develop a substitute for the ceramic packages, which is capable of overcoming the disadvantages of the ceramic packages and has advantages of conventional plastic packages such as high productivity and high economical efficiency. As a result of these attempts, and then air cavity type plastic packages have been developed. The development of air cavity plastic packages have been made possible due to the fact that as cutting-edge technologies used only for military purposes have been commonly used in non-military sectors and their reliability standards has been made less strict, a plastic sealing technique required for formation of air cavity type packages has been proposed. Methods for manufacturing air cavity type plastic packages are being rapidly developed in order to meet the demand of mass production with low prices.
In addition to the fundamental difference between the ceramic packages and the conventional plastic packages that the ceramic packages are of air cavity type, the ceramic packages have one more advantageous difference compared to the conventional plastic packages. That is, the ceramic packages can be easily designed to make less noise and dissipate heat generated from operating electrical or electronic devices more effectively. Since air cavity type plastic packages as well as the conventional plastic packages cannot realize the above advantage peculiar to the ceramic packages, the air cavity type plastic packages cannot extensively replace the ceramic packages yet. In other words, it is nearly impossible to design air cavity type plastic packages to function as well as the ceramic packages do using a current air cavity plastic package technique, and even when this is accomplished, the air cavity type plastic packages may not have the advantages peculiar to normal air cavity type plastic packages, such as high productivity and high economical efficiency.
In the case of the conventional plastic package, its plastic body is formed after mounting a semiconductor chip on a lead frame. On the other hand, in the case of the air cavity type plastic package, its plastic body is previously formed on a lead frame and then a semiconductor chip is installed into the plastic body. Thus, when manufacturing the air cavity type plastic packages, the major elements comprising each package, that is, a plastic package base and a cover, must be prepared before packaging a semiconductor chip, in other words, before mounting and wire-bonding a semiconductor chip on a lead frame. Next, a semiconductor chip is mounted on the package base and then the cover is bonded to the plastic package base with an epoxy adhesive, thereby forming an air cavity in which the exit and entry of air can be completely blocked.
In the meantime, according to the kinds of plastic and an adhesive, air cavity type plastic packages are divided into two different types including an air cavity plastic package having a base and a cover which are injection-molded using a thermoplastic resin and are bonded to each other with a B-stage epoxy, and an air cavity plastic package having a base and a cover which are transfer-molded using a thermosetting resin and bonded to each other with an A-stage epoxy. The latter is considered to be better than the former in view of productivity and reliability.
There is a conventional sealing method used to manufacture an air cavity type plastic package, in which covers whose adhesion portions are coated with the B-stage epoxy in a solid form are hardened by arranging the covers on a base through the use of a sophisticated machine, pressing the covers with a predetermined weight, and keeping the covers in a closed oven for a long time. This method takes advantage of the characteristics of the B-stage epoxy that when heated, the B-stage epoxy is liquefied and becomes very sticky at the early stage and then is rapidly hardened after the heating process is over. Even if expanding air in a cavity being gradually sealed with the B-stage epoxy forms a hole in the B-stage epoxy and is exhausted from the cavity, the hole in the B-stage epoxy can re-seal itself because of its characteristic that if heat is applied to the B-stage epoxy, it becomes sticky.
However, the conventional sealing method for forming an air cavity type plastic package has several problems. First, although the B-stage epoxy is expensive, its physical strength or adhesion strength is low, thus reducing the sealing reliability of the B-stage epoxy. Second, forming an air cavity type plastic package following the conventional sealing method is complicated and the operational efficiency is very low because during coating each cover with the B-stage epoxy and hardening the B-stage epoxy, each of the covers must be pressed with a predetermined weight or a predetermined pressure and it takes a long time to harden the B-stage epoxy.
There is another sealing method for forming an air cavity type plastic package using a liquefied A-stage epoxy as an adhesive. The liquefied A-stage epoxy, unlike the B-stage epoxy, does not have a self-curing function, and an air cavity cannot be sealed with the liquefied A-stage epoxy through a typical oven-hardening method. However, the A-stage epoxy is relatively economical and has strong adhesiveness.
The real functions of an air cavity type plastic package can be performed on a package base. In most cases, a metal ground plate acting as a heat plate is attached to the bottom portion of an air cavity type package in order to effectively dissipate heat generated from operating a semiconductor chip, and leads are installed to hang over predetermined surface areas of the metal ground plate and are a predetermined distance apart in order to manufacture the air cavity type package to make less noise.
FIG. 1 is a cross-sectional view illustrating a conventional air cavity type plastic package formed with the use of the A-stage epoxy. Referring to FIG. 1, a package base includes lead frames 12, a plastic body 14 constituting the walls of an air cavity and formed on the lead frames 12, under the lead frames 12, and at the sides of the lead frames 12, and a heat dissipation/radiation plate or a heat sink 10 bonded to the plastic body 14 with an adhesive 16. A chip 18 is installed on the heat plate 10 and is wire-bonded to each of the lead frames 12 with bonding wires 22. A ground line 24 connects the chip 18 and the heat plate 10. A cover 20 is bonded to the plastic body 14 and the lead frames 12 with a liquefied epoxy adhesive 28 over the package base. There is an exhaust port artificially formed through the cover 20. The exhaust port is closed with a stopper 26. Thus, the air cavity type plastic package having an air cavity formed therein around the chip 18 is completed.
The process of sealing the air cavity type plastic package shown in FIG. 1 is as follows. The chip 18 and the bonding wires 22 are bonded to the package base, and then the liquefied epoxy adhesive 28 is put on contact portions between the cover 20 and the package base. Next, the cover 20, the exhaust port of which is not closed with the stopper 26 yet, is placed on the package base. After that, heat is applied to the liquefied epoxy adhesive 28 so as to harden the liquefied epoxy adhesive 28 and fix the cover 20 to the package base with the epoxy adhesive 28. Next, heated air in the air cavity type plastic package is exhausted through the exhaust port for a while, and then the exhaust port is closed with the stopper 26 formed of a liquefied epoxy adhesive. Finally, the air cavity formed in the air cavity type plastic package is completely sealed by hardening the liquefied epoxy adhesive forming the stopper 26.
However, the conventional method for forming an air cavity type plastic package described above has several problems. First, it is difficult to prevent or remove mold flash from being generated during the process of forming the walls of an air cavity of plastic and subsequent process, and thus the whole air cavity type plastic package may be contaminated with the mold flash. Secondly, in inserting the adhesive 16 into a space among the heat plate 10, the lead frames 12 and the plastic body 14 for bonding the heat plate 10 to the plastic body 14, if the amount of the inserted adhesive differs even minutely from a predetermined amount, the sealing ability of the bonding interface between the heat plate 10 and the plastic body 14 may be deteriorated.
Hereinafter, problems with the conventional air cavity type plastic package will be described greater in detail. The most effective way to prevent mold flash from occurring on a lead frame in a molding process is strongly pressing the lead frame with the upper and lower dies of a mold. However, it is impossible to employ this method for preventing occurrence of mold flash during a process for forming the walls of an air cavity. In addition, even though a complicated process for removing mold flash which has already been generated by melting the mold flash with chemicals and removing the mold flash with the use of water pressure is performed, it is difficult to completely remove the mold flash. In order to make a package more compact, the walls of a cavity must be formed as narrow as possible, and the amount of an adhesive used to bond a heat plate to the plastic body must be very small. A liquefied epoxy having a high viscosity and having strong adhesiveness with a metal heat plate is used as the adhesive. It is very difficult to reproducibly apply a fine amount of such a liquefied epoxy to the plastic body. If the amount of the liquefied epoxy applied to the plastic body is slightly excessive, the liquefied epoxy may spread to the effective area of the heat plate and thus it may become difficulty to bond the ground line 24 to the heat plate 10. On the other hand, if the amount of the liquefied epoxy applied to the plastic body is slightly insufficient, the sealing ability between the plastic cavity walls and the metal heat plate may be considerably deteriorated, and thus the quality of the whole plastic package may deteriorate.
In the conventional method for manufacturing an air cavity type plastic package, in order to bond the cover 20 to the package base and close the exhaust port of the cover 20 with the liquefied epoxy, high-priced and sophisticated machines must be used to apply a liquefied epoxy to the package base and the exhaust port of the cover 20. However the production per hour of the conventional method for manufacturing an air cavity type plastic package is very low in terms of the demand of such high-priced machines, and thus it can be said that a considerable amount of investment in a plant and equipment is required. In addition, in order to make the artificial exhaust port of the cover 20, mold dies for the cover 20 must be accurately machined are prone to be rapidly worn out. Through the worn-out portions of the mold dies, a thin resin layer as mold flash may spread over the exhaust port during molding the cover 20 and shut the exhaust port, thereby lowering the productivity.
FIG. 2 is a cross-sectional view illustrating another conventional air cavity type plastic package. Referring to FIG. 2, a package base includes a first lead frame 42 and a second lead frame 40. The first lead frame 42 includes a pad on which a chip 48 will be mounted. The first plastic body 44b mold-shaped with the first lead frame 42, under the first lead frames 42, and at sides of the first lead frame 42 form the walls of an air cavity. A second lead frame 40 including a lead acting as an external terminal and a heat plate is bonded to the bottom surface of the first lead frame 42 with conductive welding flux. Second plastic body 44a is mold-shaped with the first lead frame 42 and the second lead frame 40 to form the package base. The chip 48 is bonded to the central pad of the first lead frame 42 and is wire-bonded to the lead of the first lead frame 42 with bonding wires 52. A cover 50 is bonded to the package base with an adhesive 46. Then, the formation of the air cavity type plastic package, in which an air cavity is formed around the chip 48, is completed.
The air cavity type plastic package is formed by forming inner walls of the air cavity with the first plastic body 44b firstly mold-shaped while pressing the first lead frame 42 with the upper and lower dies of a mold, bonding the second lead frame 40 including a heat plate and a lead to the bottom surface of the first lead frame 42 with conductive welding flux, and then forming the outer walls of the air cavity with the second plastic body 44a secondly mold-shaped. However, since the walls of the air cavity become thick after being subjected to two molding processes for forming the first and second plastic bodies 44b and 44a, the air cavity type plastic package cannot meet the demand of making a package more compact. In addition, the conventional air cavity type plastic package has some more problems in which adhesive strength at the interface between the first plastic body 44b and the second plastic body 44a is very weak, the number of molds increases, and development expenses are considerable.